The present disclosure relates to a technology of measuring starting positions of two scanning lines to be simultaneously formed on a photosensitive member.
An image forming apparatus of an electro-photographic type has been known which has a light scanning apparatus that deflects light beams emitted from two light sources by a rotary polygon mirror to form two scanning lines on a photosensitive member at the same time. The light scanning apparatus is provided with a sensor that detects the two light beams. Based on a detection timing at which the sensor detects one light beam, writing timings in a main scanning direction of the two scanning lines are determined.
For example, however, positions of the two light beams incident onto a mirror surface of the rotary polygon mirror are deviated in a rotating direction of the rotary polygon mirror, so that the timings at which the two light beams are illuminated to the photosensitive member may be deviated each other. Regarding this, as described above, in the configuration in which the writing timings of the two scanning lines are determined based on the detection timing of one light beam, the starting positions of the two scanning lines on the photosensitive member are deviated, so that a quality of a formed image may be deteriorated.
Regarding the above problem, a multi-beam laser scanning apparatus has been suggested which alternately lights up two light beams to measure a deviation amount of starting positions of two scanning lines based on a time difference between a detection timing of one beam and a detection timing of the other light beam in a sensor and thus corrects the starting positions of the two scanning lines so as to suppress the deviation amount.